The numbers presented by Pew help describe current patterns, but they don’t tell the whole story of which denominations are most attractive to people who might be looking for a new one. Gain and loss numbers can wind up skewed by how large (or small) different groups are to start off with. It’s easier for unaffiliateds to gain followers, on net, because there are fewer unaffiliateds to lose in the first place.

In fact, people raised unaffiliated have about the same chance of staying unaffiliated (53 percent) as people raised Catholic do of staying Catholic (59 percent), according to the Pew data. But when unaffiliateds lost about half of their 9.2 percent share of U.S. adults, they lost only 4.3 percentage points’ worth of the population at large. In contrast, because nearly a third of Americans are raised Catholic, when that church loses about half its members, it amounts to a much bigger group of people.

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Why do evangelicals wind up ahead of other Christian sects in this model? They’re better at holding on to the people born into their tradition (65 percent retention compared to 59 percent for Catholics and 45 percent for Mainline Protestants), and they’re a stronger attractor for people leaving other faiths. According to Pew’s data on conversion rates, 10 percent of people raised Catholic wind up as evangelicals. Just 2 percent of people born as evangelicals wind up Catholic. The flow between mainline and evangelical Protestants is also tilted in evangelicals’ favor. Twelve percent of those raised evangelical wind up in mainline congregations, but 19 percent of mainline Protestants wind up becoming evangelical.

One thing I didn’t get the chance to tackle, which did interest me when I was looking at the results, was that the two groups that have the easiest time pulling members from a lot of other faiths are the ones that have, in general, the lowest barriers to entry. Unaffiliateds, by definition, are the path of least resistance–you “join” just by leaving the church you currently belong to. The Unaffiliateds include atheists, agnostics, and religious people who just aren’t connected to any particular church — for some people joining the Unaffiliateds might not involve a theological shift at all.

Evangelical Protestants, the other big recruiters, differ a bit in how they welcome new converts in individual denominations, but they’re closer, in general, to the “pray the Sinner’s prayer and c’mon in!” model. You can become a member (and know that you’re a member) on your first encounter in a lot of traditions.

In contrast, Catholicism and Orthodox Christianity are harder to join. Catholic converts tend to attend RCIA classes for about half a year (and, if you come in at the wrong point in the cycle, you might wind up waiting a year for classes to restart. The Orthodox don’t have as formal a process, but my friends who have converted have tended to study with a priest (either alone or in a group) until both the priest and the aspiring parishioner agree that the person trying to convert knows what they’re getting into.

It’s actually even tougher for Orthodox than Catholics or Evangelicals (I would guess) because their numbers are lower. It’s less likely that someone seeking will know an Orthodox person or have an easy time finding their way into a church. Here in DC, I’ve got four or so Catholic churches within easy walking distance of my downtown apartment. There’s not a single Orthodox parish in the city I can reach via the metro.

The numbers in my analysis aren’t good news for Catholics, and one strategy to think about is making it easier for potential converts to walk in and get started, not loosening the pedagogical aspect of conversion, but removing small inconveniences where possible. Another option is thinking more about tailored pitches to seekers from particular theological backgrounds (just as Scott Hahn and Mark Shea have written a lot about what confused them as Protestants approaching the church) to try to make it easier for honest questioners to consider Catholicism in the first place and get the chance to notice if it happens to be true.